Testing instrument for transmitters



Aug. 25, 1931. A. D. RING TESTING INSTRUMENT FOR TRANSMITTERS Filed Aug. 22. 1929 Fig.1.

2a 40 so aoma zoo mmsn VOLTAGE on cumzzmmma PER CENT Invehtori Andrew D. Ring, by I His Attorney.

' Patented Aug; 25, 1931 AH'JJREW .ni mam, or: walammom :omrmcm 0r conmmm,

nmmmc sonny, areonrommxou 01? NEW YORK 'TFQHHG I ImVHm FOR TRANSMITTERS a A il-mam flat a; a2, 1929. 54:11:11 No. 387,738.

The present invention relates toa testing instrument'for radio'tmnsmittirfiappamtus and the like and more-particular y it relates to an instrument of this character which is adaptedpri arily for measuring ths cent-age moghiiation of a transmitter. 4

V In adjustinga transmitter it is imperative that a means of measuring pereenta 1110611 Iation available, soihat: the calm iiity' of thetransinitters to handle high modulation and the; capability of the modulators tijPItU clues suitable audio voltsges be checkerfg In theoperation of a. transmitter a percentage modulation lindi'cationisne'cessary t0 adfjust theinput-to -thes ech ampfifiem I I At present,v osm 'loseopes are used almost BXCiHSiVGiY in test measurements of transmit fer operation. Aeoil is eoupfec} to t'he tank circuit as the transmitter; the pithfi gp current is rectified by means of it twoement vacuum tubearrd supplies? to the vibrator coil of the oscilloscope." The beam is justed snthat with no pick-up it 190% modulation, a dithe amount 03E pick; up is varied until with noymal carrier and: no modulationthe Beam strikes on zero elation f the carrier is modulate cf, the beam osci-Ilafe s baeksand 'forth across the screen, pmciucin 1 an image" which agpear s to be a olid straiglit lithe. j The lengt' of this line compared with the length pf this l me with no modulation onis portrongi to the envelope of carrier 1 anci therefore indicates percentage of mothziatfon. r For laboratory useithe'osoiflogruph is mdispensable. It has many applications, and is quite accurate bat there are SEVEIgfl ab es; tions to its use in broadcasting stations and field Work. There ai e our characteristics that are orciinari'ly taken in ggerating anti adjusting atransmitfen (T) oniforing or measuring the per cent modulation: isponsrderedtoh the mostinipogtant. ({Qi) 'ifhe (W61:- f'reqaency charaoteristie anti (3) ampli- I tnde characteristic measurements are also fin portant'; (4 The Carrier than es timing modulation are" important. in ng'eerhpiste: adjustments on, atmnsmitte'r.

In obtaining the overall? frequency hiilii teristi-o, as is weii known, the inpuf, rn'ochr- Iaiion frequency at a constant voltage is ried, and the percentage modulation of the transmitter is observed, and ffiilj'c'orlefi oplitfiGil this frequency variation should n t Likewise, in

affect the percent modulation. obtaining the amplitutie' characteristic of transmitter the percentage modulation is observed, whiie the input or modulation voltage is varied at a constant frequency. Foicorrect' operation the percentage modulation plotted a aim-st the input voltage should he a, straight fine. a

The osciilograph cannot be "used for taking irequeney characteristics the vibrating element will not respond. to the high frequency. For monit ring there are several Qbjectitins to the oseillogra h. Fir'sfait is di'fiicult to read during rapid modulation and I enc'ed o erators seldom read the modulation correct y without s ecial effort. It cannot be read except in one position of the head.- The instrument is delicate and if the Vibratingelement becomes defective it f eqliifes cOns1 gable experience to re air it. Next, th oscilloscope is" not similar 50; any of the standand equi Ifint and is Ciifiic'fil'fi to mount. For am hm characteristics it can only be used at ow fgequene 1000 cycles or below)s T116 Object) 0' iis invention is to provide an iII'l' raved testinginstrum nt for measuring t e operating charaeteristiegof a radio transmitter and particularly the per ent modulation; A, further object; of this inventiOnis t6 provide atte ting instrument for radio "transmitters which is a dapfied for use in in broadcasting stati n in the field, ane'f which may provided as a unitin the panel equipment of a station. 2 a

All of the required tests for a transmitter may he made with the modulation indicator oft'he present" invention; It is well suited for measuring; the Cent modulation, freqllefiy characteristics, amplitude characteristics and. changes In; the carrier duringv moiulation. For monitoring it operates in the same manner as a Volume, indicator in a studio control room, in fact,fiii be ealis I (TU; 0; deei bels" (6Z5) so that itagrees exactly with the brated in transmission units control room measuring devices. For ampiiexperi;

Assmxoit 'ro GENERAL sion unit above mentioned will-hereinafter be referred to by the letters TU' although as is well understood the abbreviation or symbol is v10'times the logarithm of that ratio as is well known.

a The invention will be better understood from the following description when con-- sidered in connection with the accompanying drawings and its scope will be. pointed .out in'the appendedclaim'sj'f In -t'he drawings, Fig. 1 isffla wiringdia-x gram of a testing apparatus embodyingithe invention; Fig. 2 is a curve'diagramshowingthe operation of oneof'the electric discharge devices. represented in Fig.1; and Fig. 3 is a curve diagram showing the range of control of the apparatus shown in l. 'Referring to Fig. .1, 5 is a pick-up coil'in which Voltage from a 'transmitter may ?be in-1 input circuit forthe testing apparatus or 7 ti-fier 15 thus assured of being a smooth enduced when using the instrument' fo'r testing. The'coil may be placed externally to the reinaind'er of the apparatus and is connected therewith by suitable leads 6. The pick-up coil istuned to the carrier frequency of the transmitter by suitable means such a vari-, able co'ndenser7. The pick-up coil and the tuning'means 7 provide a tuned pick-up or instrument I Connected withthe tuned inputcircu-it by leads 8 and 9. are a pair of branchor parallel: connected circuits 10 and 11." In circuit' 10 is connected an audio-frequency:choke coil 12 and an average current responsive indicating device such as a direct current milliainmeter' 13. In circuit 11 is connected a direct current blocking means such asfa. condenser 14 of.a capacitysuch that' it is adapted to con vey alternating, current of a low or audio.

frequency andan impedance '15 in the form of a tapped resistor across which a How of audio; frequency current through circuit 11 sets up alternating" current voltage, This circuit niay also include 'a device 16 for in",- dicating the value of the alternatin'gjor audio frequency current in circuit,11.'

'Interposed between the junction of the parallel branch circuits 10 and name-{the tuned input circuit for the apparatusis a rectifier device,17 and meansfprovidinga tuned. trap circuit 18 inser es w th QRIQSIQJGQI 19. The trap circuit includes suitable. tuning;

means which in thepresent example are indi catedas a fixed'coil or inductance2011mm shunt variable condenser 21.

In the arrangement ofth 'ple which is the preferred arrangement of the apparatus, the tunedtrap circuit 18-and 'The'detector 17 includes a cathode22 and (lb could be used as well. The number of TU; corresponding to the-ratio of any two powers,

The'catliodeof the rectifier is heated by suitable means such as a battery 24. In series with thebatteryisasuitable filament control rheostat 25 and invv shunttherewith is .a potentio neter 26 throughv which the low potential'end of circuit 11 isreturnedto, lead 8, said low, potential end beingconnected with a moving Contact on the potentiometer.

Followii' g .therectifier 17 and not including thetuned circuit18 andresist or- 19, is a reservoir condenser shunted across leads 8 and9- whereby it receive's and stores energy from the input circuit afterbeingrectified by device t l. n-lVith thisarrangement; current from a transmitter maybe induced in pick-upcoil 5 and the tank circuit 5,1 6, 7 which'is tuned to the frequency .of the'jtransmitter. Current from thetanlgior tunedcircuit is suppliedthrough the rectifiertubeinto the reservoir c-ondenser'QS. The c0ndenserf28 becomes charged on the peak current and sustainsfilts charge practically throughout the negative half cycl e.;ij.The output of the rec velope of the input radio;;frequency. v

Any -carrier'frequency which is carried over through the rectifier: to leads 8. and 9 is trapped. out, byj the ftu 'ied trap circuit 18. The resistor lfi serve'sto limit the load which is placed Yon the tuned input circuit, and also assures that the output of the rectifier will. be a linear function'of theinput.

FIOlHlZhQ foregcing description it will be seen thatthe radio freque-ncyor carrier current which is pickedsup in the tunediinput circuit and rectifiedin device .17, is suppliedthrough tuned-trap circuitQl S and resistor 19to a pair of branch or parallel circuits 10 a dfllmi whichi iltl has a low, impedance to the direct current component .of the rectified current while the other, llihas a. low :iinpedance t'o the audiojor alternating current con ponentof such rectified current.

J The separation of the direct current and audio freeuiency' components of the rectified current is. effected in th-e branch circuits by means; of the, choke coil" 1-2 and the condenser 14, The choke coil .blocks the audio frequency cur n Wh h tends to fiow'from direct currenujjwhile the condenser blocks the direct current andgpasses the audio frequency .cnrrent through branch 11. The current in the'direct current branch is measurediin thecircnit of the present example, by means of the milliammeterliland the current-in the'audiofrequency branch maybe measured byasuitable indicating device or milli'ammeter 16.. However, this last is not practicable in the instrument of the present example as will be explained hereinafter.

Thecurrent in the direct current branch is proportional to the carrier voltage, there fore, theunilliammeter or indicating device 13 in this circuit may be usedto indicate whether modulation is more positive than meter.

The resistance of the alternating current oraudio frequency branch Circuit between leads Sand '9 is made equal to the resistance of choke coil 12 so. that the ratio of the: cur-' rents in each branch will also be in proper tion to thevoltage supplied from the input circuit, and rectifier 17 between leads 8 and 9. It is evident that with 100% modulation negative- Thus it will serve as aldistortion of the carrier-current, the peak values of di-,

rect current and audio frequency voltages will beequal, thus the effective audio frequency current as indicated on a milliammeter 16 will be i ofthedirect current. It the two currents" are known, percentage modulation can easily be computed by inultlplylng the audio current measured by milliammeter 16 by.

and dividing ,theresult bythe direct current. If thedirect-currentcomponent is held constant," the percentage modulation is directly proportional to. thecurrent in the audio branch. Measuring thedirect current in branch 10 is, very "simple, as a milliammeter of appropriateinaximum current rating such as that mentioned; is all that is required.

Measuring the current in the audio fre-f quency branch 11 is more complicated, however, as alternating current milliam-meters such as 16 are; too sluggish in their action to followthe variations in audio frequency amplitude. Accordingly the impedance or resistor is. placed inthe audio frequency to an enlarged scale.

The grid of connection 32 on tapped resistor 15 whereby the alternating current voltage applied to the grid may be varied. This voltage is that existing between lead 8 or low potential side of the branch circuits and the point on resistor 15 to which the grid is connected. The vacuum tube voltmeter is provided with an anode circuit 33 including a source of anode current 34, an indicating device or direct current milliammeter 35 and an audio frequency choke coil 36. The anode circuit is also provided with a suitable by-pass condenser 37: The current in the alternatingv current or audio frequency branch 11 is now measured by means of the milliammeter 35 in the plate circuit 33 of the voltmeter tube 30. This tube is preferably one having a high amplification constant and consequent large plate current variation for small variations in grid voltage. .This vacuum tube voltmeter must be calibrated in the usual manner as is wel known, before it can beiused. I

As indicated on the diagram, a constant bias potential is supplied to the grid of the voltmeter tube from the filament voltage drop in the potentiometer 26, although it may of course be supplied by any other suitable means. The amount of this bias potential is adjustable by moving the sliding con tact 27 of the potentiometer so that it will permit an anode current flow in the voltmeter or indicator circuit 33 sufficient to bring the indicator or milliammeter 35 to' an initial or low reading. r

In Fig. :2, to which attention is new directed, the'curve A-B is a part of the static grid characteristic of the voltmeter tube plotted The oint which the indicator 35 reads when at justed with no modulation (no'A. C. grid voltage on the tube) corresponds 'to' 0.2 milliamperes of anode current with a Radiotron UX811 tube, which is that used at-30 in the circuit represented in the present example. This is point B on the curve and requires four volts of bias'to be reached. The line N N is drawn through this point and represents the point about which the grid Voltage swings when the vacuum tube voltmeter is excited.

As a test on an apparatus substantially as shown and described, a cycle alternating current voltage was impressed on the grid of device 30in addition to the direct current bias and adjusted until it'caused the plate current to rise from .2 to 1.1 milliamperes. Thiswas measured and found to have a root mean square value of four volts. This corre- A curve This wave is I is read on the value or .318 I That is, for a peak value ofbuilt of standard parts and mounted in a con- 3.5 milliamperes the average value of current, as is read on indicator should be 1.113 milliamperes. The current indicated in the anodecircuitwas 1.10 milliamperes. Thus the static and constructed dynamic characteristics agree remarkably well, showing that the vacuum tube as applied in this instrument is independent of frequency, even though the frequency be carried very high. The slightdifference inresults are probably due to the divergence from true sine wave loops of the plate current curve.

7 Two courses are openin arranging the tube voltmeter in this instrument. One isto tap the grid'lead from a fixed point ofthe resistor 15 and to read audio frequency current on the indicator or milliammeter 35. The other is to make this point variable as shown and hold the anode current of the voltmeter tube '30 constant. The latter method is preferred, as it allows the indicatoror meter to beoperated over the more accurate portion of its scale. 1

. In this arrangement, since the grid'voltage is always adjusted for the same anode current, the grid voltage swing is the same at all percentages of modulation within the range of the indicator, when the tapped resistor 15 is properly adjusted. The RI drop in the resistor 15 bet-ween the grid and ground thus being constant, the current will vary inversely as the resistance included by the tap.

The resistances to be included between each I tap and low potential lead 8 .are calculated in the following manner: (Assuming the current in the direct current branch 10 to be held constant at ten milliamperes for example,)

the currents in the audio frequency branch and found to be four volts. These values of resistances are tapped off the wire-wound tubular resistor 15. In operation, the ta switch 32 is adjusted until the anode current of the tube 30 is at its required value and the TU up or down is determined from the posi-.

tion of the tap switch. This switch may be,

. arranged to read percentage modulation dicontrol apparatus.

rectly in steps, such as ten per cent steps, but it is advantageous to have it operate on integral TUs so that it may be similar to audio A curve for converting TU up or down to percentage modulation or vice versa is shown in Fig. 3. From this curve which is self explanatory, it can be seen that the range covcred by seven taps as indicated in Fig. 1, from 12 TU to 2 TU, corresponds to a percentage modulation range of from 25% to 125%.

The modulation indicator is adapted to be trolrack of a transmitter as one of the units thereof. It is compactand the only external part is the pick-up coil, which may be of small dimensions. 7 Having. only meters,

switches, and dials in its constructiomit may be made to conform to the generalappearance of any station control apparatus. '7

The cost is much less than that of the oscil loscope which has heretofore been used. -Repairs may be made as on any of the remote control equipment whereas it requires considerable skill to repair the vibrator of an oscilloscope. v

The indicator may serve five purposes; that of monitoring, indicating percentage modulation, taking frequency and amplitude characteristics, and indicating variation'i-n'the carrier during modulation (that is, unsymmetrical modulation)- Taken as a whole, the indicator or testing instrument is cheaper, more accurate for monitoring and a smaller and more practical means of measuring percentage modulation than is the oscilloscope. What I claim asnew and desireto secure by Letters Patent of the United States; is

1. An electrical apparatus adapted for testing the modulation of a radio transmitter, said apparatus including a tuned input circuit comprising a pick-up coil and means for tuning said. coil to a desired frequency, a pairof branch circuits connected in parallel with said tuned input circuit, one of said branch circuits including an audio frequency choke coil and a direct-current milliammeter connected in series, the other of said circuits including a blocking condenser and, a'potentio-meter connected in series, a tuned trap circuit and a resistor connected in series between one junction of said branch circuits and said tuned input circuit, a rectifier device connected between the other junction of said branch circuits and said tunedinputlcircuit, anda vacuum tube *voltmeter including an electric discharge device having a grid provided with a variable connection to saidpotentiometer, means providing avariable bias voltage for said electric discharge device, and an anode circuit for said device including an audio frequency choke coil, a direct current milliammeter and a sourceof anode current, said milliammeter and saidpotentiometer being calibrated to indicate a comparison of the audio frequency and direct current components of current supplied from said input circuit through said rectifier device to said branch circuits. V

2. A testing instrument for a. radio transmitter including an input circuit, means for tuning said input circuit to a desiredfrequency, a direct current branch (circuit, an audio frequency branch circuit, indicating meansfor reading current flow in said direct current circuit, a vacuum tube voltmeter for reading current flow in said audio frequency branch circuit, a variable resistor in said audio frequency circuit for applying voltage to said vacuum tube voltmeter, and a tuned trap circuit and a rectifier device inserted in circuit between said branch circuits and said tuned input circuit.

In witness whereof, Ihave hereunto set mv hand this 20th day of August, 1929.

ANDREW D; RING. 

